A variety of electronic devices utilize a rechargeable battery. Examples of (a) the rechargeable battery include a lithium ion type, a nickel-cadmium type, and a nickel-metal hydride type; and (b) the electronic devices include laptop computers, cell phones, personal digital assistants, power tools, etc. Some rechargeable batteries may become hazardous under certain conditions including and not limited to over voltage conditions, over current conditions, thermally unacceptable conditions, some other unacceptable battery or electrical circuit condition, or combinations thereof. Hence, a variety of battery protection circuits have been utilized in battery packs of such rechargeable batteries.
FIG. 1a is a schematic diagram of a prior art battery protection circuit used in association with a rechargeable battery pack 1. The rechargeable battery pack 1 has a power cell 3, such as one or more lithium ion cells, that normally provides power to a load 2. The load 2 and the cell 3 are electrically connected through an upper supply rail, +Ve, and a lower power supply rail, −Ve.
Use of the term “battery” pack is not intended to indicate that more than one power cell 3 is necessarily employed, although in various embodiments, more than one power cell 3 may be employed.
The load 2 can be a passive load such as a cell phone or a PDA, or an active element such as a battery charger, which can recharge the rechargeable battery pack 1. As seen in FIG. 1a, the rechargeable battery pack 1 includes the power cell 3, a thermal protector 4 and a protection circuit module (PCM) 5 driving the load 2. An arrow 16 shows the direction of positive current flow in a loop that includes the power cell 3, the load 2, the PCM's electronic switching devices 7 and 8, and the thermal protector 4. When the load 2 is supplanted by a charger, the direction of positive current flow 16 in the loop is reversed.
The PCM 5 includes an integrated circuit control chip 6 operatively coupled to one or more electronic switching devices 7 and 8. The electronic switching devices 7 and 8 may be any variety of transistors including field effect transistors (FETs). The FETs can be, for example, a metal oxide semiconductor field effect transistors (MOSFETs), as illustrated in FIGS. 1a and b, or bipolar junction transistors. The PCM 5 is essentially a switch that (a) detects abnormal current or voltage and (b) disconnects, shortly after the switch detects the abnormal current or voltage, the cell 3 from the load 2, or, alternatively, a charger if the rechargeable battery pack 1 is being charged.
The thermal protector 4 provides protection for the rechargeable battery pack 1 from thermally unacceptable conditions. Thermally unacceptable conditions can damage or impair electronic components such as those in the load 2. The thermal protector 4 may be, for example, a thermal fuse, a thermal breaker or a positive temperature coefficient (PTC) thermistor. Thermal protector 4 may also be either non-resettable or resettable. Non-resettable thermal protectors have lower equivalent series resistance (ESR), but once tripped, a rechargeable battery pack employing the non-resettable thermal protector is essentially no longer of any use. Resettable thermal protectors have higher ESRs, but can be tripped and reset many times.
ESR is a parameter that determines the usable energy stored in the cell 3, and thus the usable energy stored in the rechargeable battery pack 1. Generically, lower ESR means longer operation such as longer talk times for a cell phone. In the rechargeable battery pack 1, the ESR thereof includes the internal resistance of the cell 3, the resistance of the thermal protector 4, the resistance of the electronic switching devices 7 and 8 (as illustrated), and the resistance of any connectors and other conductors in the circuit path 16 (a) to the load 2 and (b) from the load 2. Since the thermal protector 4 is in the circuit path 16 coupling the cell 3 to the load 2 and the resistance of the thermal protector 4 is not negligible, the thermal protector 4 adds to the ESR of the rechargeable battery pack 1.
The circuit control chip 6 is not in the circuit path 16 of upper and lower power supply rails, +Ve and −Ve, that includes the cell 3, the thermal protector 4 and the load 2. Instead the circuit control chip 6 is on a negligible power supply rail, denoted as Vn, and as a result the circuit control chip 6 does not significantly contribute to the ESR.
FIG. 1b illustrates an alternative embodiment wherein the thermal protector 4, illustrated as a positive temperature coefficient thermistor, and the circuit control chip 6 are both on the negligible power supply rail, Vn, and as a result the circuit control chip 6 and the thermal protector 4 do not significantly contribute to the ESR.
Under normal operation for the electrical circuitry illustrated at FIGS. 1a and b, the electronic switching devices 7 and 8 are “closed,” that is, in a conducting condition in which each device 7 and 8 can conduct current along path 16.
The over voltage/over current detecting PCM 5, for FIGS. 1a and b, and the thermal protection circuitry 4, just for FIG. 1b, are operatively coupled to the electronic switching devices 7 and 8 such that the over voltage/over current detecting PCM 5 and the thermal protection circuitry 4 can each independently cause the electronic switching devices 7 and 8 to assume an “open” or non-conducting condition, in which the electronic switching devices 7 and 8 will not conduct current. When one of the electronic switching devices 7 and 8 is open, the loop 16 that includes the power cell 3, the load 2 and the electronic switching devices 7 and 8 is effectively made open, thereby preventing (a) the power cell 3 from supplying current to the load 2 or (b) the load 2 from recharging the power cell 3.
The two above-identified rechargeable battery pack circuits that protect the rechargeable battery pack from thermal runaway, over voltage or over current conditions are disclosed and illustrated in LiTingTun's U.S. published patent application number 2009/0179618, which was published on Jul. 16, 2009. LiTingTun's circuitry fails to provide a second layer of protection. In many applications, there is a requirement for a second layer of protection to be in place in the event the main circuit fails to isolate the rechargeable battery pack.
It is understood that when the second layer of circuit protection activates, the results are typically deemed permanent. An example of a second layer of circuit protection is set forth by Denning in U.S. Pat. No. 7,667,435, which issued on Feb. 23, 2010. In that patent, Denning disclosed a block diagram, illustrated at FIG. 2, of an electronic device 100 having a DC power source 104 and a battery pack 1 to supply power to the passive load 2. If the DC source 104 (e.g., an AC/DC adapter) is not present, power may be supplied to the system from the battery pack 1. If the DC source 104 is present, it may supply power to the load 2 and provide power to recharge the cells 3 of the battery. In a battery charging mode, first switch 7 may be closed and second switch 8 may be open in one instance. In that instance, current may then flow through closed first switch 7 and a second diode 8a in parallel with open second switch 8 to provide charging current to the cells 3. In another battery charging mode, both first and second switches 7 and 8 may be closed to reduce losses due to the second diode 8b. In a battery supply mode, first switch 7 may be open and second switch 8 may be closed in one instance. Current from the battery cells 3 to the load 2 may then flow through closed second switch 8 and first diode 7a in parallel with open first switch 7. In another battery supply mode, both first and second switches 7 and 8 may be closed to reduce losses due to the first diode 7a. 
The battery pack 3 may also include a primary battery protection circuit 54 (which may include an equivalent of the over voltage/over current detecting PCM 5 and/or an equivalent of the thermal protection circuitry 4—previously described with respect to FIGS. 1a and 1b), a filter 31, a secondary safety circuit 32, and a fuse element 33. The primary battery protection circuit 54 may monitor a number of conditions including the voltage level of each cell 3 as well as charging and discharging current levels and provide charge (CHG) and discharge control signals (DSG). The voltage level of each cell 3 may also be monitored by the secondary battery protection circuit 32 via the filter 31. The filter 31 serves to filter out short duration over voltage transient spikes. The secondary battery protection circuit 32 monitors the voltage level of each cell 3 and provides a signal to the fuse element 33 to blow or open the fuse element if a voltage level of one cell 3 is greater than an over voltage threshold level for a sustained time interval. The filter 31 therefore serves to stop the fuse element 33 from blowing due to short duration over voltage transient spikes. The secondary battery protection circuit 32 is designed to provide an output to a fuse element to permanently disable the fuse element in response to a sustained over voltage condition. Denning's fuse is a voltage activated fuse since it responds to overvoltage spikes.
As described above, current circuit designs typically use voltage activated fuses and/or positive temperature coefficient thermistors (also called resettable fuses or polymeric positive temperature coefficient devices). Voltage activated fuses have a distinct advantage of passing a lot of current and requiring a low voltage signal, typically less than three (3) volts, to open the fuse and therefore isolate the battery pack from the output connectors. Such fuses tend to be small and fit into most electrical circuit systems.
There are disadvantages of using voltage activated fuses, however. One disadvantage is that few manufacturers make such a voltage activated fuse. Another disadvantage is the manufacturers of voltage activated fuses do not allow voltage activated fuses to be incorporated into products used in military applications or class III medical applications. Class III medical devices include and are not limited to pacemakers and defibrillators. One reason those voltage activated fuse manufacturers may impose such application restrictions is due to the fact that if the battery has a low capacity and that low capacity limits the amount of current it can source then the battery may fail to fully open the voltage activated fuse. Alternatively written, the voltage activated fuse may not open completely if the battery cannot supply enough power and thus leaves a closed fuse that was supposed to be open. Such fuse failure would be deleterious to a patient, military operation, or other application. In view of these known disadvantages, there is a need to design a voltage activated 2nd level safety circuit for permanent isolation that uses a current activated fuse. Second level safety circuits are designed to protect against first level protection circuit component failure. For example, when a first level protection circuit component detects an over voltage condition, an over current condition, a thermally unacceptable condition or some other unacceptable battery or circuit condition and the electronic switching device(s), normally a field effect transistor, fails to open and/or is fused closed when that unacceptable condition is detected, then the second level safety circuit is activated and permanently opens the battery pack.